Usually, it is true, each species produces only its like,--"every creeping thing and beast of the earth" bringing forth young "_after his kind_." But the development of a single animal, under the ordinary law, takes place in a few weeks or days; while the development of distinct races and species is the work of a whole creation, and is spread over countless ages. It is reasonable to suppose, that the latter is effected by means of a higher law, manifesting itself only at long intervals. Its infrequent manifestation is no argument against the regularity and necessity of its occurrence,--against its being a law at all. The comet that visits our system only once in five hundred years is controlled by the same inflexible principle which causes the return of another comet once in five years. The conditions requisite for a development more perfect than usual,--that is, for the production of a new species,--instead of a new individual of the same species, may be fulfilled only at long intervals; but when they are fulfilled, the result--the more perfect development--takes place as necessarily, as much by the virtue of law, as the more ordinary phenomenon of the propagation of one race. These conditions may be answered in the successive stages of improvement, through which the earth and its atmosphere pass, during the vast periods of time contemplated in geology. In the era of the old red sand-stone, for instance, there were no higher animals than fishes, because the atmosphere was highly charged with carbonic acid, and could not support respiration by lungs. When the air became purer, the gills were changed into the imperfect lungs of the amphibious tribes, such as the huge saurians and the frogs. Deprive these latter animals, in their lower stage, of all access to the light, and they will not advance to their higher stage. Put a tadpole into a perforated box, and sink it to the bottom of a river, and the animal will never be perfected into a frog; he will grow to an enormous size, but he will continue a tadpole.